1. Field of the Invention
The present invention relates to engine valve lifter mechanisms of an internal combustion engine, and more particularly to the engine valve lifter mechanisms of a type that is incorporated with a variable valve control device that can adjust a valve lift characteristic of the engine valves, such as, intake valves and/or exhaust valves.
2. Description of the Related Art
Hitherto, various engine valve lifter mechanisms of the above-mentioned type have been proposed and put into practical use particularly in the field of wheeled motor vehicles powered by internal combustion engines.
One of such engine valve lifter mechanisms is disclosed in Japanese Laid-open Patent Application (Tokkai) 2006-57637. The engine valve lifter mechanism disclosed in the Laid-open Application is applied to a reciprocating internal combustion engine and is of a direct operated type installed between a stem end of an intake valve and a rotation cam mounted on a cam shaft.
The engine valve lifter mechanism generally comprises a cylindrical lifter body that is slidably received in a cylindrical guide bore formed in a cylinder head, a circular head that is integrally mounted on an upper end of the lifter body and having an upper surface contactable with the rotation cam, and a cylindrical boss that is projected downward from a center part of the circular head and contactable with the stem end of the intake valve.
The circular head is formed at two given portions thereof with respective oil holes for permitting oil on the upper surface thereof to flow into the interior of the lifter body thereby to apply the oil to a friction generating section between the boss portion and the stem end and a valve spring that is operatively installed in the lifter body.
The oil holes are so positioned that a bearing pressure applied from the rotation cam to the upper surface of the circular head when the rotation cam passes across the oil holes does not exceed a maximum bearing pressure applied to the upper surface when the rotation cam passes across areas other than the oil holes. For this positioning, the two oil holes are placed at diametrically opposed portions of the circular head. That is, during a lift period when the rotation cam causes the intake valve to take an open operation, the bearing pressure exhibits the maximum value at a generally center area of the upper surface of the circular head (viz., the area from which the cylindrical boss is projected downward) and exhibits the minimum value at outer peripheral portions of the circular head due to inevitable layout of the cam shaft on the cylinder head. Thus, the oil holes are arranged at the outer peripheral portions of the circular head where the bearing pressure is sufficiently small.
With the above-mentioned arrangement of the oil holes, increase in the bearing pressure in the vicinity of the oil holes in the period when the rotation cam contacts and presses the circular head is sufficiently controlled, and thus, undesired oil film break on the upper surface of the circular head, which causes deterioration of lubrication, is suppressed. Thus, abrasion of the peripheral edge of each oil hole is suppressed or at least minimized.